The field of single electron devices emerged from investigations of the tunnel junction, which consists of two electrodes of a conducting material, separated by a thin layer of an insulating material having a thickness of about one nanometre. According to the laws of quantum mechanics, electrons have a small probability of tunnelling through such an insulating layer. If a voltage is applied across the junction, electrons will tunnel preferentially in one particular direction through the insulator. Hence, they will carry an electric current through the junction. The magnitude of the current depends on both the thickness of the insulating layer and the material properties of the conducting electrodes.
In early 1985, Averin and Likharev attempted to predict the behaviour of very small tunnel junctions with superconducting electrodes but the equations were too complex to be easily solved. However, for a small tunnel junction with electrodes of normal conductors, if a constant electric current is passed through a junction, it will induce a voltage that oscillates periodically in time. These periodic oscillations have a frequency equal to the current divided by the charge of an electron. This frequency is totally independent of any other parameters of the system. An alternative view is that each oscillation represents the response of the device as a single electron tunnels through the insulating layer. The phenomenon is known as single-electron tunnelling (SET) oscillations.
To understand this effect, one must appreciate how electric charge moves through a normal conductor such as an aluminium wire. An electric current can flow through the conductor because some electrons are free to move through the lattice of atomic nuclei. Despite the motion of the electrons, any given volume of the conductor has virtually no net charge because the negative charge of the moving electrons is always balanced by the positive charge of the atomic nuclei in each small region of the conductor. Hence, the important quantity is not the charge in any given volume but rather how much charge has been carried through the wire. This quantity is designated as the xe2x80x9ctransferredxe2x80x9d charge. This charge may take practically any value, even a fraction of the charge of a single electron. The reason for this is that charge is proportional to the sum of shifts of all the electrons with respect to the lattice of atoms. Because the electrons in a conductor can be shifted as little or as much as desired, this sum can be changed continuously, and therefore so can the transferred charge.
If a normal conductor is interrupted by a tunnel junction, electric charge will move through the system by both a continuous and a discrete process. As the transferred charge flows continuously through the conductor, it will accumulate on the surface of the electrode against the insulating layer of the junction (the adjacent electrode will have equal but opposite surface charge). This surface charge Q may be represented as a slight continuous shift of the electrons near the surface from their equilibrium positions. On the other hand, quantum mechanics shows that the tunnelling can only change Q in a discrete way: when an electron tunnels through the insulating layer, the surface charge Q will change exactly by either +e or xe2x88x92e, depending on the direction of tunnelling. The interplay between continuous charge flow in conductors and discrete transfer of charge through tunnel junctions leads to several interesting effects. These phenomena can be observed when the tunnel junctions are very small and the ambient temperatures are very low. (Low temperatures reduce thermal fluctuations that disturb the motion of electrons.) In this case, if the charge Q at the junction is greater than +e/2, an electron can tunnel through the junction in a particular direction, subtracting e from Q. The electron does so because this process reduces the electrostatic energy of the system. (The energy increases in proportion to the square of the charge and does not depend on the sign of the charge.) Likewise, if Q is less than xe2x88x92e/2, an electron can tunnel through the junction in the opposite direction, adding e to Q, and thus again decrease the energy. But if Q is less than +e/2 and greater than xe2x88x92e/2, tunnelling in any direction would increase the energy of the system. Thus, if the initial charge is within this range, tunnelling will not occur. This suppression of tunnelling is known as the Coulomb blockade.
If the surface charge Q is zero initially, then the system is within the Coulomb blockade limits, and tunnelling is suppressed. Therefore, the current flowing from the source through wires will start to change the charge Q continuously. For convenience, assume that the deposited charge rate is positive rather than negative. If the charge reaches and slightly exceeds +e/2, tunnelling becomes possible. One electron will then the junction, making its charge slightly greater than xe2x88x92e/2. Hence, the system is within the Coulomb blockade range again, and tunnelling ceases to be possible. The current continues to add positive charge to the junction at a constant rate, and Q grows until it exceeds +e/2 again. The repetition of this process produces the single-electron tunnelling (SET oscillations: the voltage changes periodically with a frequency equal to the current divided by the fundamental unit of charge, e.
To produce SET oscillations, tunnel junctions must be of a very small area and cooled to ensure that the thermal energy does not influence tunnelling. Typically, the device must be cooled to temperatures of about a tenth of a degree above absolute zero if the junction is 100 nanometres in length and width.
European Patent Application EP 0 750 353 discloses a single electron tunnel device of this invention which includes a multiple tunnel junction layer including multiple tunnel junctions; and first and second electrodes for applying a voltage to the multiple tunnel junction layer, wherein the multiple tunnel junction layer includes an electrically insulating thin film and metal particles and/or semiconductor particles dispersed in the electrically insulating thin film.
The electrically insulating thin film may be made of an oxide and the particles may be of at least one type of metal selected from the group consisting of gold (Au), silver (Ag), copper (Cu), platinum (Pt), and palladium (Pd).of the particles. Their diameter may be 50 nm or less.
Fabrication of suitable structures to support single electron tunnelling has proved difficult. In particular, it has proved difficult to form films having the size and disposition which are suitable for tunnelling. However, we have now devised a method suitable for the fabrication of arrays of these devices.
According to one aspect of the present invention there is provided a single electron tunnelling device comprising a particle of a material having a first conductivity characteristic having a surface layer of a material of a second conductivity characteristic, the thickness of said layer being sufficiently small to support quantum mechanical tunnelling therethrough together with first and second electrodes positioned adjacent to said particle to facilitate the flow of current therebetween.
Said first and second electrodes may be superconducting.
In a preferred embodiment of the invention a plurality of such particles is positioned between said first and second electrodes.
There is also provided a method of fabricating single electron devices comprising the steps of forming a plurality of particles forming a layer of a thickness sufficiently small to support quantum mechanical tunnelling on the surface of said particles and positioning at least one of said particles between a pair of electrodes to form a single electron device.